(1) Field of the Invention
This invention relates to a process for preparing a bicyclic trienone, useful as an intermediate for the total synthesis of steroids and analogs thereof, by dehydrobrominating a brominated bicyclic dienone.
(2) Description of the Prior Art
5-Ethenyl-4,4a,7,8-tetrahydro-4a-methyl-2(3H)-naphthalenone, having the structure: ##STR1## is described by Ruppert et al., Chem. Ber. 106, 3636-44 (1973), as a byproduct obtained in the condensation of the bromoethylidene compound of the formula: ##STR2## with the sodium salt of 2-methyl-1,3-cyclopentanedione. The main products of the reaction were alkylation products of the cyclopentanedione. The bromide of formula II was in turn prepared by reaction of 5-ethenyl-4,4a,5,6,7,8-hexahydro-5-hydroxy-4a-methyl-2(3H)-naphthalenone of the formula: ##STR3## with phosphorus tribromide in methylene dichloride solution.
Valenta U.S. Pat. No. 3,920,748, issued Nov. 18, 1975, describes a process for the total synthesis of steroids in which the initial step is a Diels-Alder reaction between a 2,6-di-lower-alkylbenzoquinone and a substituted 1-vinylnaphthalene compound. One of the suggested intermediate 1-vinylnaphthalene compounds is the structure given in column 13 of the Valenta patent, labeled IIIb and having the structure: ##STR4## WHICH IS THE SAME AS FORMULA I given above. The patent states that the structure IIIb can be prepared by dehydration of the vinyl alcohol of structure XXXI, e.g. ##STR5## by treating the latter with freshly fused potassium bisulfate and pyrogallol at 145.degree. C. at reduced pressure, although no specific example of this conversion is given. The last paragraph of Example 12 of the patent gives the experimental details of a dehydration of the corresponding vinyl alcohol lacking the endocyclic double bond.
Cohen U.S. Pat. No. 3,920,749, issued Nov. 18, 1975, describes a process for the dehydration of 1.beta.-hydroxy-9.beta.-methyl-1-vinyl-polyhydronaphthalen-6-ones using a Lewis acid such as boron trifluoride etherate to produce a 9.beta.-methyl-1-vinyl-polyhydronaphthalen-6-one with a double bond in the 1,2-position of the naphthalene ring. Example 1 of the patent gives the experimental details of the dehydration of 1.beta.-hydroxy-9.beta.-methyl-1-vinyl-perhydronaphthalen-6-one of the formula: ##STR6## to give 9.beta.-methyl-1-vinyl-3,4,5,6,7,8,9,10-octahydronaphthalen-6-one of the formula: ##STR7##
Dehydrohalogenation is a process wherein the elements of halogen and hydrogen are removed from an organic molecule to form a carbon-carbon double bond. A wide variety of reagents have been used to effect this transformation. Lithium chloride or bromide, lithium carbonate or a combination of lithium carbonate and lithium bromide have been used for dehydrobromination of steroid alpha-bromo ketones; cf. Fieser and Fieser, Reagents for Organic Synthesis (John Wiley & Sons, Inc. 1967), Vol. 1, pp. 604-609. Pyridine has been used as a dehydrohalogenating agent, for example in the conversion of 2-(.alpha.-chloroethyl)thiophene to 2-vinylthiophene; Emerson et al., Org. Syn. Coll., Vol. 4, 980 (1963).